1. Field of the Invention
This invention relates to an improved hollow fiber of high molecular weight, and to a method of manufacturing the same which involves preparing a spinning solution with a non-polar solvent, extruding the solution, and simultaneously extruding a neutral aqueous salt solution from an orifice encircled by the spinning solution being extruded, so that a phase separation takes place by means of a salting out effect.
2. Description of the Prior Art
Recently, semipermeable hollow fibers have been used for blood dialysis, for fluid separators based on the principles of ultrafiltration, and reverse osmosis. Such a hollow fiber should have a uniform configuration because when used, for example, for blood dialysis, the pressure in the interior of the hollow fiber is usually positive compared to that at the exterior. If the hollow fiber is not uniform in thickness, there is a possibility that the hollow fiber may be broken at its thinnest part. Also, it is known that when the cross section of the hollow fiber departs significantly from that of a true circle, blood is apt to coagulate in the hollow fiber.
When the hollow fiber is used for reverse osmosis, high pressures up to 100 atmospheres may be applied to the outside of the hollow fiber. In this event, it is necessary that the cross section of the hollow fiber should approximate a true circle and that the thickness of the wall of the hollow fiber be uniform. Otherwise, the hollow fiber can become crimped and useless. If the cross section of the hollow fiber is not uniform, the polarized concentration due to nonuniform flow substantially reduces the permeability performance of the fiber.
One procedure for the manufacture of hollow fibers involves extruding a spinning solution from an annular slit of a double pipe orifice, forming a sheath solution, and simultaneously, a gaseous or liquid fluid is extruded from the inner pipe of the double pipe orifice to form a core fluid. Where the manufacturing method uses a gaseous material as a core fluid, the resulting hollow fiber is apt to be crushed because the direction of the filament after being spun is changed by means of a guide bar in the coagulation bath and the washing bath. Thus, the running filament may be strongly pressed on the guide bar and likely to become deformed.
These disadvantages are substantially reduced by using a melt spinning procedure. However, it is known that using this procedure makes it difficult to produce a hollow fiber having a favorable selective permeability.
In using a wet spinning process, the spinning conditions such as concentration, coagulation bath conditions, and the like can be widely varied. On the other hand, it is difficult to produce a hollow fiber of uniform shape and productivity of this type of process is usually very low because of the low speed of spinning.
In the method in which a core liquid is extruded instead of a gaseous core fluid, the deformation of the running filament at the guide bar is considerably avoided. However there may be another problem developed, that is, a rapid coagulation of the sheath spinning solution develops by the instantaneous diffusion of the core liquid into the sheath dope immediately after being spun. Specifically, a thin skin layer is first formed on the inner interface of the spun sheath in contact with the core liquid, and another thin layer is formed on the outer surface of the sheath by contact with the coagulation bath liquid when the running filament is introduced into the coagulation bath. These two layers determine the dimensional configuration of the hollow fiber, and subsequent coagulation between the two layers usually develops numerous macro-voids in the membrane of the hollow fiber. These voids serve to scatter light and, as a result, the resultant hollow fiber looks whitish. A desired selective permeability cannot be obtained from such a fiber because the membrane has unfavorable macro-voids instead of desired micro pores. Furthermore, the spinnability is very poor because of the rapid coagulation of the spinning solution immediately after being spun. The hollow fiber, which is obtained at a very slow speed, on the order of 15 meters per minute at the most, has very poor mechanical properties. Also, the skin layers formed on both the inner and outer surfaces of the hollow fiber lower the permeability.
A dry jet-wet spinning method in which the spinning solution is extruded into a gaseous space and then is introduced into a coagulation liquid is considered to be preferable for producing hollow fibers having good permeability characteristics. In this method, a spinning solution or dope is extruded from an annular slit to form a sheath, and the extruded sheath passes through a gaseous space before being introduced into a coagulation bath. The gaseous space may be filled with an inert gas or air, and it may contain the vapor of the solvent of the spinning solution.
The core liquid is preferably non-coagulative at least before the spun solution is introduced into a coagulation bath. For example, in the production of cellulose hollow fibers by the cuprammonium process, the core liquids may consist of materials such as benzene, toluene, trichloroethylene, n-hexane, or perchloroethylene, which are not miscible with water. In this type of procedure, the organic core liquid must be removed from the hollow fiber in succeeding steps, which involves long, troublesome operations. Furthermore, from the standpoint of pollution problems, the use of such toxic organic solvents is not favored.
When the hollow fiber is to be used for medical purposes such as in blood dialysis and for any process involving foods, for example, such as the concentration of juice, the above mentioned toxic organic materials should be completely washed out from the hollow fiber.
In the patented art, U.S. Pat. No. 3,799,356 describes the manufacture of hollow fibers from quaternized hollow fiber membranes of polymers containing a sufficient amount of a vinyl pyridine to be non-thrombogenic.
U.S. Pat. No. 3,930,105 describes another method for continuously extruding hollow fibers from an acrylonitrile polymer in which the polymer is extruded or dissolved in a highly polar organic solvent.
Other types of hollow fibers and methods for their manufacture are described in U.S. Pat. Nos. 3,933,653; 3,944,485; and 3,674,628.